2 weighings. All the balls are to be divided into three groups: two 3-ball groups and 2 balls in the third group. First, weigh both 3-ball groups. If one of them outweighs the other, take any two balls out of it and weigh between themselves. If they’re equal, the third ball is the one you need. If the first weighing showed both groups were equal, weigh the two remaining balls — one of them will be heavier.